This invention relates to crystalline copolymers of .epsilon.-caprolactone and glycolide. More specifically, it relates to crystalline copolymers of .epsilon.-caprolactone and glycolide prepared in a controlled two stage polymerization process which are suitable for the preparation of surgical articles, especially monofilament sutures.
The preparation of surgical articles from synthetic, bioabsorbable polymers by molding or spinning fibers is well known. The most widely used polymers for the preparation of such articles are derived from hydroxy acids and their condensation products, anhydrous cyclic esters commonly referred to as lactones. Among the most widely studied lactones for preparing bioabsorbable polymers for surgical articles are .epsilon.-caprolactone, lactide and glycolide. Naturally, recent attention has been focused on improving the physical and biological properties of polymers and copolymers derived from these specific lactones.
The most recent attempt to optimize the properties of copolymers of .epsilon.-caprolactone and glycolide is disclosed in U.S. Pat. Nos. 4,605,730 and 4,700,704. These patents describe single and two stage polymerization processes for preparing the copolymers. Of particular interest is the two stage polymerization process. Specifically, the patents disclose in a number of different examples first preparing a low molecular weight prepolymer of .epsilon.-caprolactone and glycolide, and then polymerizing in situ the resulting prepolymer with glycolide to produce a crystalline copolymer.
Unfortunately, the crystalline copolymers prepared in the two stage process described in these patents do not provide the most desirable physical and biological properties when fabricated into surgical articles. Most significantly, when the copolymers are spun to prepare fibers exhibiting an acceptable biological profile for surgical sutures, the fibers are quite stiff. This stiffness, or lack of compliance, makes it difficult to tie knots from sutures prepared from these fibers, and additionally, reduces the knot integrity when knots are tied because of the tendency for the knot to become loose. Furthermore, the physical properties of fibers prepared from these crystalline copolymers, e.g. straight and knot tensile strength, or their biological properties, e.g. in vivo breaking strength retention (BSR), may not be adequate to meet the requirements for a suitable surgical suture in some surgical procedures.
In view of the generally less desirable compliance characteristics of crystalline copolymers of .epsilon.-caprolactone and glycolide, useful surgical articles, and in particular monofilament surgical sutures, from such copolymers have not been developed. Typically, the lack of compliance of fibers spun from such copolymers would require the braiding or twisting of individual fibers to prepare a multifilament suture. Multifilament sutures are disadvantageous because, among other things, they often require a surface coating for enhancing the lubricity and smoothness of the suture surface to prevent trauma to tissue during suturing.
Therefore, in view of the deficiencies of the prior art, it would be most desirable to prepare a copolymer of .epsilon.-caprolactone and glycolide which can be fabricated into useful surgical articles. In particular, it would be desirable to prepare such a copolymer which can be spun into fibers, and exhibit outstanding compliance as well as excellent physical and biological properties for the preparation of monofilament sutures.